1. Field of the Invention
This invention relates to active optical systems and more particularly to an active optical system for steering an incoming laser beam.
2. Description of the Related Art
Many types of optical systems require the steering of a propagating laser beam. When an image or a digitally encoded optical beam propagates through, for example, turbid media, random density fluctuations cause changes in the local index of refraction. This can lead to step-like changes in the index of refraction that a propagating beam may experience, causing a change in propagation direction, akin to the change that occurs when a laser beam enters a piece of glass. In telecommunications applications, laser beams that are exiting an optical fiber may need to be steered into one of several other optical fibers. If the beam propagation direction is not switched quickly enough, transmission will be interrupted causing a loss of data transfer. In each of these situations, a steering system can be used to return the beam to its original propagation path. Standard steering systems rely on the mechanical motion of mirrors. These systems can be made quite reliable, but are limited in their application. This is because a mechanical system has inherent speed limitations.
An early mechanical system is described in U.S. Pat. No. 4,157,861, entitled Optical Beam Steering System, issued to Kirk E. Davies. This patent discusses the use of an electromagnetically driven reflective surface on a carefully mechanically optimized mount to minimize vibration. Since the steering is performed by moving a mirror, the steering speed is limited by mechanical motion bandwidths.
An improvement on the mechanical steering concept is addressed in U.S. Pat. No. 5,067,829, entitled Dynamic Optical Beam Steering, issued to Jaskie, et. al. In this patent, optically transparent elastic materials are applied in thin films to a substrate. Application of an electric field causes mechanical distortion of the material the beam to change its propagation direction. Although an improvement, this patent still describes a mechanical system (since the layers have to physically move), thus resulting in limited speed.
Faster steering can be realized using electro-optical means to steer the beam. There have been previous patents that disclose the use of electro-optical means to perform rapid beam steering. An early electro-optic steering device is described in U.S. Pat. No. 5,420,875, Steering of Laser Beams, issued to Sternklar. In this patent, beams are steered by a third-order nonlinear optical process, the steered beam interacting with a nonlinear polarization induced in a nonlinear medium by two other pumping beams. The steering effect is much faster than mechanical means, but is limited to the narrow angle defined by the pump beams. Additionally, the use of a third-order effect means that two pump beams of relatively high intensity must be used, since third-order effects are very weak.
A more recent example is described in U.S. Pat. No. 5,943,159, entitled Method and Apparatus for Optical Beam Steering, issued to Zhu. In this patent, an optical device is spatially addressed via an array of electronic connections. By manipulating the local phase of the beam, the propagation direction is changed and the beam is steered. This requires an elaborate optical array. The nature of the physical effect limits the steering to small angles. U.S. Pat. No. 6,137,926, entitled Hybrid Optical Multi-Axis Beam Steering Apparatus, issued to Maynard, discloses a mechanical system, that is much miniaturized so that it can reach speeds higher than traditional mechanical deflectors. However, it is still limited by mechanical speeds.
The present invention is an active optical system for beam-steering an incoming laser beam. It includes a first control optics assembly for receiving an incoming laser beam and adjusting that incoming laser beam in accordance with first desired wavelength, polarization and beam propagation parameters. A driver element produces a driver laser beam. A second control optics assembly receives the driver laser beam and adjusts that driver laser beam in accordance with second desired wavelength, polarization and beam propagation parameters. A second order non-linear optical element (SONOE) receives an output from the first control optics assembly and an output from the second control optics assembly. The SONOE provides a non-linear optical interaction between the outputs such that a propagation direction modified laser beam is provided that propagates at a different angle than the incoming laser beam and at a difference wavelength thereof. Egressing optics receives the propagation direction modified laser beam and adjusts that laser beam in accordance with desired wavelength, polarization, and beam propagation parameters, the output of the egressing optics having the laser beam propagation direction shifted relative to the incoming laser beam direction. The present invention is all-optical, so the steering speeds are much higher than those achievable with mechanical steering. Since it results from a second-order effect, the nonlinear effect can be performed with only one driving beam instead of two. Since the physical effect that steers the beam is difference frequency generation, the phase-matching rules allow the steered beam to be directed to an almost any direction.